Racecar Vehicle Dynamics by the Milliken Brothers:
Front-biased CG creates a bigger slip angle at the front than the rear, causing understeer. The car must compensate for the difference in slip angles in order to follow a more "neutral" path. There's more but I'll post later...
Bring back wider rear wings, V10s, and tobacco advertisements
Agreed and well said. I did not dare to enter into slip angle...
The thread also lacks the explanation of apparent increment of rotational inertia: turning the car around the curve opposes the rotation of the car around its own axis.
The torque generated by the rear wheels to move the center of mass around the curve, tries to turn the car over its own axis "out" of the curve, incrementing the "yaw moment". So you have two things to analyze independently: the rotation of the car on a circle and the rotation of the car over its own vertical axis (yaw): the smaller the radius, the bigger the "effective" rotational inertia of the car. This effect alone would turn a perfectly static balanced car into a dynamically under steering car.
Finally, the rotation of the wheels creates a gyroscopic moment that increases the "yaw inertia", although this is a tertiary effect that can be significant in high speed curves.
These are the reasons why:
* The rear wheels are wider than front wheels and also
* Most sports cars have a slightly heavier rear axle.
What a normal driver "sees" is a little hesitation of the car in the entrance of the curve.
I have argued for better track design, in part because this "hesitation" occurs right when the transition of the super elevation (that occurs in the straights, at the entrance of curves in old roads), is trying to slid your car sideways into the curve, with an opposite effect, which that the pilot has to figure out.
What a normal driver sees here is that the car “pulls” into the curve while you are going in the straight before the curve, and corrects unconsciously (check it next time you see somebody driving in an old road: there is a little anti-curve movement of the steering wheel right before the curve), but I am starting to sound like Jeremiah, prophet of the old testament...
I love this stuff. The more you examine it, the more you have to learn.
There is also the factor of gyrocsoic precession. If you turn the wheel hard, the presession pushes the wheels to lean towards the inside of the turn. And if we have four wheel steering, then you could dial in slip angles based on velocity, steering wheel angle, and gas or brake pedal position.
Also, on a conventional front steering setup, the front wheels generate slip angle and heat before the rears. Once they get heat, they generate more grip.
Thanks for your replies,
but the question is still open.
Where is the optimal position of the CG in longitudional direction.
Try to imagine you are constructing a race car and you can place the
weight absolutely free.
Your car has rear wheel drive and the rear wheels are a bit larger
in width than the front wheels.
Furthermore you want to accelerate through the corners like in Formula1.
Can you give me some numbers like 40-60 or 30-70
and how you got them calculated.
Simply put, there isn't an optimum location. That will vary depending on where and how the car needs to be fast on the track how the car works and which track it is on and what the weather is doing also how big the difference in tyre size between front and rear and which end is driven.
Long fast corners, you might want the weight to spread the load better between the tyres. Big acceleration zones, you might want the weight shifted back to help traction. Can't get the front to bite on turn-in you may want the weight forward.
Like all of these things the engineers have to look at each track and set of circumstances to have the CofG where they think tey will find most time.
A very powerful car may be able to give up some corner speed if you can get it accelerating for long and get the power down earlier. A puny little car with no power would probably benefit from close to 50/50 weight distribution, but even then if it is traction limited (wheelspins) on parts of the track there may be time found in getting weight over the driven wheels.
Weight over the rear tyres not only helps traction, but lets you use more rear brake before rear wheel lockup; I wouldn't be surprised if an F1 benefits from moving the weight back until it is detrimental to front end bite.
You want the traction in the front to be atleast equal to that of the rear so you dont understeer. So for an all around, general fast car for the road, the optimum place to concentrate the mass would be somewhere behind the midline of the wheelbase but not on the rear wheels themselves, in my opinion. You dont want to concentrate your mass towards the front, or in the middle, because as you brake and enter a turn the load transfer will not only put more weight on the front inside tire, but even more on the front outside, and the more forward your weight is, the more weight that will be put on those tires. The trick is to balance the design of the suspension as in roll moments, and try to have them as close to the CG line longitudinally as possible. And although your roll moments will change, you dont want them to change that much. And obviously designing optimum suspension components helps with the whole thing for various reasons which i dont feel like writing. But suspension links play a huge roll in all of this, as in where they are placed and there relative length. Way too much stuff to go into.
